Link to redirect creation page when editing a version (#632)

This is implements the idea 4) from
readthedocs/readthedocs.org#11978 as a first
step.

However, we can get fancier here and automate this creation somehow if
we want. There are more ideas for UX in that issue.

---------

Co-authored-by: Anthony <[email protected]>

Author: humitos

readthedocsext/theme/templates/projects/version_form.html

@@ -53,7 +53,11 @@ <h2 class="ui medium dividing header">
           <div class="content">
             {% if version.active %}
               <div class="ui warning message">
-                {% trans "Deactivating a version or changing its slug will result in the deletion of its documentation." %}
+                {% url "projects_redirects_create" project_slug=project.slug as url_redirects %}
+                {% blocktrans trimmed with url=url_redirects %}
+                  Deactivating a version or changing its slug will result in the deletion of its documentation.
+                  Make sure to <a href="{{ url }}" target="_blank">create a redirect</a> for these URLs to ensure old links continue working.
+                {% endblocktrans %}
               </div>
             {% endif %}